This invention relates to screens or screen cylinders more particularly to screens which are used in pressure screening apparatus for removing contaminant particles from papermakers' stock.
High turbulence pressure screening apparatus of the kind shown, for example, in U.S. Pat. No. 4,155,841 issued May 22, 1979 to Chupka and Seifert, and assigned to the same assignee as this invention, is used in the preparation of papermaking stock. A suspension of liquid and paper fibers, which may in varying degrees contain undesirable rejects or contaminate particles, is supplied to the inlet of the apparatus as disclosed, for example, in the '841 patent, where it is applied to an annular cylindrically shaped screen having specifically designed slots or perforations therethrough. Typically, the paper stock is fed to the interior of a vertically oriented cylindrical screen, and the rejects are withdrawn from one end of such a screen, while the accepts pass through the slots or perforations in the screen and are collected at a location outwardly of the screen. Rotating foils or vanes are positioned usually on or adjacent the inside of the screen surface, in close relation to the surface, to reduce the plugging of the screen slots or holes.
Typically, the screen cylinder of the kind described above is formed by machining the required slots or grooves in a flat plate of metal material, such as stainless steel, and then by rolling the material into the shape of a cylinder, and welding the rolled plate at the abutting ends to form a welded seam. Thereafter end rings, formed from bar stock, are rolled, welded at their respective ends, and then attached by welding to the cylinder plate, at its respective ends. Further, one or more intermediate reinforcing rings, also formed from rolled bar stock, are attached by welding at longitudinally spaced locations on the outer surface of the rolled screen plate. An example of such a fabricated cylinder screen is shown in Hatton et al, U.S. Pat. No. 4,017,387, issued Apr. 12, 1977 and assigned to the same assignee as this application.
As described in Hatton et al, such cylinders are subject to fatigue failures. The fatigue failures are believed to be induced principally by the dynamic pulse pressures of the rotating foils usually carried by a rotor assembly and positioned to run in close relation to the inside surface of the cylinder. These foils, which may rotate at speeds at 5000 ft. per minute or more create, in the cylinder, a moving positive pressure wave along the inside face followed by a relatively negative pressure behind the foils.
At least two foils are employed with the result that the screen is subject to a radial outward bending at a number of rotationally moving axially extending locations, followed by forces forming an inward bending or negative pressure at the intermediate locations. The positive and negative pressures may be considered as placing the material alternately in compression and tension.
The stress condition, described above and also described in Hatton et al, is exacerbated by the fact that all of the components making up the screen cylinder originally began as longitudinally flat components which were rolled and welded to a cylindrical form and in many cases to each other. The rolling process inherently creates, in any such flat bar or plate material, a condition of compression on an inside diameter and tension at an outside diameter. While, in the manufacture of such cylinders, steps are taken to relieve such stresses, nevertheless it is believed the stresses cannot be fully relieved while maintaining adequate hardness and desired crystal structure of the metal, and it is believed that residual stresses are inherently retained in the finished screen cylinders.
Further complications reside in the fact that openings, in the form of holes, relieved areas, or slots, are cut or formed in the sidewall plate while in the flat condition, and become inherently distorted by the rolling process. Further, since a large number of slots are formed in closely spaced relation, each usually terminating at common positions on the plate to form rows of such slots separated by land areas, the rolling process is inherently nonuniform circumferentially of the plate due to the differences in strengths between a slotted and a non-slotted region. It is accordingly believed that stress concentration points tend to be formed at the terminal ends of the slots during rolling due to differential bending at these locations.
The welding along the plate seam and along the end rings and the intermediate rings inherently creates lines of weakness where a weld may not be as strong as the original material. Also such a weld will commonly exhibit a lower tolerance to fatigue cycles than the basic material. In any case, catastrophic failure of such screen cylinders in service are most commonly found associated with one or more of the welds such as at a welded seam or at a ring. Further, a high percentage of failure modes include fatigue cracks which extend from or to an end of a slot defining a point or region of stress concentration.
For some time, in order to reduce fatigue failures at conventional weld joints, electron beam welding techniques have been used to attach the reinforcing rings to the outer cylinder bodies. While this technique has somewhat reduced the instances of weld failures at the reinforcing rings, the weld itself is particularly difficult to inspect for integrity, and a number of failures have been attributed to imperfect joining of the adjacent parts by the electron beam.
A need therefore exists for a screen cylinder, and a method of manufacturing the same which eliminates the source of catastrophic breakage and premature fatigue-induced failures in cylinder screens.
The most costly and time consuming step in the manufacture of the screen cylinder is that of the cutting of the slots or openings in the side wall. Since this is commonly performed in the flat plate material, prior to rolling, the exact nature of the slotted pattern, including the extent of openness and the width and dimension of the slots or openings for any particular screen must be known prior to fabrication. Such plates have been damaged during the rolling process, thus resulting in the scrapping of a valuable piece. Further, fabrication by rolling and welding of the individual components inherently produces a cylinder which is not truly cylindrical, but will usually have a run out of as much as 0.125 inches in 24 inches of diameter. Such out-of-round condition not only complicates fabrication, it also adversely impacts upon the clearance which must be provided between the foil and the closest inside surface.
The practice of cutting the wall of the plate prior to rolling makes prohibitively costly the warehousing or stocking of any large quantity of pre-manufactured cylinders, and generally a cylinder is made only after receiving a specific order for the same.